Two unrelated families are described, of which four adult members were found to be suffering from severe protein C deficiency (≤ 5% of normal plasma level). Newborn deaths were reported in the first family but the second family had no history of neonatal purpura fulminans and infant death. Thrombotic symptoms developed mainly in their early twenties, consisting chiefly of recurrent superficial and deep iliofemoral vein thromboses and pulmonary emboli. Other clinical features included generalized peritonitis due to massive mesenteric vein thrombosis, thrombosis of the cavernus sinus, renal vein thrombosis and priapism. In the second family, five members (all aged approximately 40 years) died of intravascular abdominal thrombosis. Massive thromboembolic episodes were associated with a compensated disseminated intravascular coagulation syndrome as evidenced by high concentrations of D‐dimer (mean levels 5000 ng/ml plasma) and by a moderate reduction in platelet count and fibrinogen concentrations. D‐dimer levels in non‐crisis periods were also raised above normal (mean levels 400 ng/ml). The thrombotic problems of these patients were controlled satisfactorily by long‐term administration of low molecular weight heparin alone or in combination with low dose warfarin.